Can fire be made stationary?
As long as you don't need the smoke to start animating again after it has stopped you can set the start and end frame for the bake to before the set animation frames start.



Above I have set the start frame to -50 and the end frame to 1.  Once baked, the smoke animation will be done by (and freeze) at frame 1 and stay frozen for the full animation.  (For some reason the end frame can not be set lower than 1, but the start frame has no such limit.)